焼付硬化性超深絞り用高張力冷延鋼板の開発

抄録

Effects of carbon and niobium contents, and continuous annealing conditions on mechanical properties, structures and embrittlement of cold rolled steel sheets containing extra low carbon, low niobium and phosphorus have been investigated. The following results are obtained:
(1) The steel sheets with Nb/C (atomic ratio) ≤0.5 show high bake hardenability but poor drawability. The steel sheets with 0.7≤Nb/C≤1.2 exhibit excellent deep drawability and high bake hardenability originated in solution of niobium carbides by annealing at over 850°C. (2) For maintaining high bake hardenability, rapid cooling is effective especially at near 750°C and at near 300°C. (3) Cold-work embrittlement due to adding phosphorus can be prevented by means of leaving a small amount of solute carbon in steel sheets through the rapid cooling at the above temperature ranges. Critical content of phosphorus (P_er) causing cold-work embrittlement can be expressed in terms of the ageing index as follows.
P_er(wt%)=0.04·AI+0.045
(AI: Ageing index (kgf/mm²), Cooling rate:≥1°C/s)
Based on the above results, the cold rolled high strength steel sheet with bake hardenability and excellent deep drawability (r=2.3) has been produced_in a continuous annealing line.